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Matter in accretion disks spirals inward because of angular-momentum losses due to intrinsic shear torques. For galactic disks, the accretion rates can be as high as 1M⊙/yr, corresponding to a value larger than unity of the dimension-less viscosity parameter α. The required torques exceed turbulent ones by at least an order of magnitude; they can, however, be the result of magnetic tensions.
In fast-revolving, clumpy disks, magnetic pressures can exceed static gas pressures and approach ram pressures. Suggestive candidates for such magnetic dominance are the inner, near-rigidly rotating parts of galactic disks.